The present invention relates generally to the image display systems that utilize color separation prisms. More particularly, this invention relates to an improved method and configuration for assembling a color separation prism assembly compensated for contrast enhancement and implemented as reflective imager.
A technical difficulty of compound angle depolarization is still confronted by those of ordinary skill in the art in designing and assembling reflective imager type projection display systems implemented with a polarization beam splitter (PBS). Color separation prisms are placed between quarter wave plate (QWP) and imager cause undesirable phase changes at the tilted dichroic and total internal reflection (TIR) surfaces. System performance is degraded because of the poor contrast. A quarter wave plate (QWP) placed between polarization beam splitter (PBS) and reflective imager will correct compound angle depolarization. However, the process required to glue the QWP adds to the processing complexities that causes difficulties in mass production. Thin films with optical characteristics to control phase difference can be coated on the total internal reflection and dichroic reflection surfaces to mitigate the effects caused by undesirable phase changes. However, existing color separation prism assemblies, as will described below, are faced with the difficulties that the assemblies become quite massive and occupy a large volume in order to provide color separation for a third color channel.
The limitation can be understood by examining a prior art patent issued to Bryars et. al. U.S. Pat. No. 6,144,498, entitled xe2x80x9cColor Separation Prism Assembly and Method for Making Samexe2x80x9d. This patent discloses a color separation device assembled from three prisms in an arrangement that provides for air equivalent thickness adjustment for each color. The air equivalent thickness adjustment provides for the correction of deviations in any of the three prisms, as well as providing a method to correct for the chromatic aberration arising from other optical elements in the information display system. The color separation device is disclosed to improve the image quality and provides opportunities to lower the display system cost by using low tolerance components and/or plastic optical components normally having a high chromatic aberration.
As discussed in Bryars et al. the combination of dichroic filters and prisms is commonly used in color imaging and display systems as a way to separate colors or combine the primary colors into the final image. A compact optical element that accomplishes this purpose is known as a Philips prism. The Philips prism assembly is commonly known, and various uses thereof are described in U.S. Pat. Nos. 2,392,978, 3,659,918, 4,009,941, 4,084,180, and 4,913,528, the disclosures of of these patents and that by Bryars et al. in U.S. Pat. No. 6,144,498 are incorporated herein by reference.
Referring to FIG. 1A for a perspective view of a conventional Philips prism assembly 10. The prism assembly 10 includes a first triangular prism 12 and a second triangular prism 14, with a third prism 16 having at least four sides. The triangular prisms 12 and 14 are positioned with respect to each other to provide an air gap 18 at their interface. The second triangular prism 14 and third prism 16 are optically cemented at an interface 20 of these prisms. As indicated in FIG. 1, prism 12 is configured to separate red light (R), prism 14 is configured to separate blue light (B), and prism 16 is configured to receive green light (G). The light ray paths through prism assembly 10 are shown in FIG. 1. An incident ray i passes into prism 12, with a portion thereof (e.g. red light) internally reflected and emerging from prism 12 as r1. The remaining portion of the incident light passes into prism 14, with a portion thereof (e.g., blue light) internally reflected and emerging from prism 14 as r2. The remaining portion of the incident ray (e.g., green light) emerges from prism 16 as r3. A color separation into red, green and blue (RGB) is therefore achieved with this Philip prism.
For a purpose to correct for the chromatic aberration of the optical system components, U.S. Pat. No. 6,144,498 discloses a color separation prism assembly as that shown in FIG. 1B. FIG. 1B is a ray diagram for a prism assembly 30 where prism 14 is assembled with an offset with respect to prisms 12 and 16 from the nominal position. The dashed arrowhead line in prism 14 represents the physical path for a prism assembly having no deviation from the nominal design. The dashed line in prism 12 represents the nominal position for the now longer side of prism 12 forming the exit facet adjacent prism 14. The solid segment lines in prism 30 represent the actual physical path caused by the deviations in prisms 12 and 14 utilized to assemble prism 30 in the corrective offset position. The prisms are offset in one dimension whereby the parallel relationship between connected facets is maintained and the path lengths are equalized such that: (ABCD)new=(ABAxe2x80x98Bxe2x80x99Cxe2x80x98Dxe2x80x99)new. The prism 16 is displaced a distance away from the common edge with prism 14 by translating its common facet parallel to the interface. The amount of displacement for prism 16 is calculated such that: (ABCD)new=(ABAxe2x80x98Bxe2x80x99Axe2x80x9cBxe2x80x9d) new. The prisms as disclosed above all have large volume. In the Philip prism assemblies described above, compensating foils on the reflection surface are used and the third prism is cut into two and one tilted interface is added to properly control the differential phase shift incurred in optical transmission. The assemblies are made with prisms of different shapes thus requires more complex assembling processes that lead to higher production cost and lower product yield and poor reliability.
Referring to FIGS. 1C and 1D for alternate prior art color separation prism assemblies disclosed by Greenberg and Bryars that comprises three prisms of distinctly shapes. The third prism is modified such that the third prism is a mirror image of the second prism. A total internal reflection (TIR) surface for the third channel is introduced. This modified prism assembly has an advantage that the prism assembly is more compact than the regular Philip prism assembly described above. However, the modified assembly creates another problem of introducing aberrations due to addition of extra components. Corrections of aberrations through differential phase shifts through transmission cannot be conveniently achieved. Furthermore, as the modified prism assembly still requires more than one types of prisms, the increase of manufacturing costs due to processes in handling different types of prisms is still not resolved.
Therefore, a need still exists in the art to provide a new and improved color separation prism assembly to overcome the above difficulties and limitations. A simplified, more compact and more economical color separation prism assembly would be desirable to further advance the state of the art of prism assembly for color separation applications.
It is therefore an object of the present invention to provide a color separation prism assembly that is more compact in size and weight and can be more conveniently manufactured and assembled such that the difficulties and limitations encountered by the prior art can be resolved.
Specifically, it is an object of the present invention to provide a new and improved prism assembly configured with three triangular prisms of identical size and shape. These three triangular prisms are further assembled into a combined prism with internal interfacing surfaces coated with dichroic filtering film for separating light into color beams. The combined prism assembled with three prisms of same shape and size is more compact and can be more conveniently manufactured and assembled by simplified processes with reduced production cost.
Another object of this invention is provide new and improved color separation prism assembly with total internal reflective (TIR) surfaces coated with phase-shift compensation coatings on only two of these TIR surfaces to reduce the work and costs required for phase-shift compensation.
Another object of this invention is provide new and improved color separation prism assembly with three triangular prisms of same shape and size to assemble into a combined prism. The combined prism thus provides a better heat dissipation configuration cooled by a single fan for dissipating the heat generated from optical energy stored in the combined prism.
Briefly, in a preferred embodiment, the present invention includes color separation prism assembly. The prism assembly includes three triangular-shaped prisms having identical shape and size. Each of the three triangular-shaped prisms has two triangular surfaces as interfacing surfaces bonded to two identical triangular surfaces of two other prisms. At least two triangular surfaces of the three triangular-shaped prisms are coated with color separation filtering films for reflecting a first color light and transmitting a second color light therethrough. At least two triangular surfaces of the three triangular-shaped prisms are coated with a total reflective coating constituting at least two total internal reflective (TIR) surfaces for reflecting a light projected thereon. Each of the TIR surfaces further comprising a phase-shift compensating coating coated thereon. The color separation prism assembly further includes three liquid crystal on silicon (LCOS) modulating means for modulating a separated color beam color-separated and projected thereon through the color separation prism assembly. The three triangular-shaped prisms with identical shape and size are further assembled as a combined prism.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various drawing figures.